A semiconductor laser, which is an optical semiconductor element, is widely used as a light source for optical communication. In view of reduction of costs and reduction of power consumption, a semiconductor laser is demanded that is usable without performing temperature adjustment by means of a Peltier element, or the like. A quantum dot laser employing quantum dots as an active layer is known to have a small deterioration in properties in operation at high temperatures, due to the effect of discretization of energy levels, or the like due to the effect of three-dimensional quantum confinement, and is suitable for operation without temperature adjustment.    Patent Document 1: Japanese Laid-open Patent Publication No. 2011-197453    Patent Document 2: Japanese Laid-open Patent Publication No. 2005-286198    Patent Document 3: Japanese Laid-open Patent Publication No. 10-242572    Patent Document 4: Japanese Laid-open Patent Publication No. 2002-217446